Chris Mason wrote:
> On Sunday 18 May 2008, Andi Kleen wrote:
> > Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:
> > > On Fri, 16 May 2008 14:02:46 -0500
> > >
> > > Eric Sandeen <sandeen@xxxxxxxxxx> wrote:
> > >> A collection of patches to make ext3 & 4 use barriers by
> > >> default, and to call blkdev_issue_flush on fsync if they
> > >> are enabled.
> > >
> > > Last time this came up lots of workloads slowed down by 30% so I
> > > dropped the patches in horror.
> >
> > Didn't ext4 have some new checksum trick to avoid them?
>
> I didn't think checksumming avoided barriers completely. Just the barrier
> before the commit block, not the barrier after.
A little optimisation note.
You don't need the barrier after in some cases, or it can be deferred
until a better time. E.g. when the disk write cache is probably empty
(some time after write-idle), barrier flushes may take the same time
as NOPs.
This sequence:
#1 write metadata to journal
#1 write commit block (checksummed)
BARRIER
#1 write metadata in place
... time passes ...
#2 write metadata to journal
#2 write commit block (checksummed)
BARRIER
#2 write metadata in place
... time passes ...
#3 write metadata to journal
#3 write commit block (checksummed)
BARRIER
#3 write metadata in place
Can be rewritten as:
#1 write metadata to journal
#1 write commit block (checksummed)
... time passes ...
#2 write metadata to journal
#2 write commit block (checksummed)
... time passes ...
#3 write metadata to journal
#3 write commit block (checksummed)
... time passes ...
BARRIER (probably instant).
#1 write metadata in place
#2 write metadata in place
#3 write metadata in place
Provided some conditions hold. All the metadata and all the journal
writes being non-overlapping I/O ranges would be sufficient.
What's more, barriers can be deferred past data=ordered in-place data
writes, although that's not always an optimisation.
-- Jamie
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